1. Field of the Invention
The present invention relates to zoom lenses which are suitable for photographing optical systems of, for example, film cameras, video cameras, and digital still cameras.
2. Description of the Related Art
A positive-lead zoom lens in which a lens unit with a positive refractive power is disposed at a position closest to the object side has been known as an example of a zoom lens with a relatively long back focus (distance between the last surface of lenses at an object point at infinity and a paraxial image plane), and a high zoom ratio. The long back focus involves a length capable of substantially arranging a quick return mirror in a single reflex camera. Also, the high zoom ratio is a zoom ratio of 3 or higher.
When an unintentional vibration is transmitted to a zoom lens, for example, because of a camera shake due to motion of hands, an image blur may appear in a shot image. To prevent this, a zoom lens has been suggested that has a mechanism (image stabilizing mechanism) for compensating an image blur due to the unintentional vibration, to increase the quality of an image.
An example positive-lead zoom lens has a five-unit zoom lens composed of five lens units with positive, negative, positive, negative, and positive refractive powers in sequence from the object side.
Such a five-unit zoom lens performs image-blur correction (image stabilization) by moving the whole fourth lens unit or a part of the lens units in a direction orthogonal to the optical axis. (For example, see U.S. Pat. Nos. 7,196,853 and 6,025,962, and Japanese Patent Laid-Open No. 2004-226644.)
The above-listed documents discuss a zoom lens including a fourth lens unit composed of two positive and negative lens components. Either or both the lens components are moved in the direction orthogonal to the optical axis to correct an image blur.
A zoom lens for a digital single reflex camera is being desired to have a high zoom ratio and provide a shot image with a high quality. Also, such a zoom lens is being desired to have a predetermined length of a back focus.
In the zoom lens, as the lens units have greater refractive powers, the amounts of movement of the lens units to provide a predetermined zoom ratio are generally reduced, and hence, the total length of lenses can be reduced and the zoom ratio can be increased easily.
However, zooming (varying magnification) with this arrangement may cause large fluctuation in aberrations. Thus, it is difficult to obtain a proper optical performance over the entire zooming range.
Also, when the sufficient length of the back focus is provided while the zoom ratio is increased, the entire lens system at an wide-angle end may define a retrofocus lens system, causing aberrations to increase at the wide-angle end.
An example zoom lens includes a lens unit serving as an image stabilizing lens unit, so that the lens unit is shifted in a direction orthogonal to the optical axis to correct an image blur. Such a zoom lens can correct an image blur relatively easily.
However, image stabilization is not performed quickly unless the lens arrangement of the zoom lens as well as the lens arrangement of the image stabilizing lens unit moved for image stabilization are proper. Decentration aberration may occur during image stabilization, and the optical performance may be degraded.
In particular, it is difficult to quickly perform image stabilization if the image stabilizing lens unit is large and heavy. The slow-speed image stabilization may cause a failure of shooting an object that moves fast.